INTENSITY OF CHORUS EMISSIONS AND AMPLIFIED WHISTLER-MODE SIGNALS FROM VLF GROUND TRANSMITTERS

Magnetospheric whistler mode waves in the magnetosphere, both natural and man-made (such as those transmitted from Siple Station), play a key role in wave induced particle precipitation (WIPP). The critical parameter is the saturation intensity Bs of these waves, which modify radiation belt dynamics. The value of Bs also differentiates between the weak-field [1], and strong-field models [2][3], that attempt to explain wave growth and emission triggering. The peak intensities predicted by these two models may differ by an order of magnitude or more. The fundamental difference between them is the assumption of the input field Bin, which in the strong field model is required to be ≥ the trapping value, whereas the weak field model requires no assumption of Bin value and indeed it shows the same saturation value for any value of Bin, assuming no interfering signals along with Bin. Since the growth rate of the Siple pulse is a well-measured parameter, these two models can be tested by finding either the input or the output intensity of these waves at the interaction region (IR) near the magnetic equator for typical duct locations near L=4. To obtain the input Bin to the IR, we can use the signal from the Siple Station transmitter as measured in the F-region above Siple Station, Antarctica by a sounding rocket [4]. A model of the duct [5] is used to calculate the fraction of this incident wave that is captured by the duct. Analysis using the slowly-varying (WKB) approximation shows that the ducted wave falls off in strength versus altitude at about the same rate as for non-ducted waves. This finding allows satellite measurements of non-ducted Siple signals to be used as proxies for the ducte